I have lost so much time over this problem and really need some professional help. Over the last days I recorded some VHS tapes, I used ffmpeg which worked great! When I finished, I noticed that must of the recording had the usual "buzzing" from VHS. So, I started to extract the sound from the MP4 file, open it on Audacity and apply an noise filter. I used these commands:
To extract the MP3:
ffmpeg -i original-file.mp4 -vn -ac 2 -f mp3 output.mp3
To put them back together:
ffmpeg -i original-file.mp4 -i new-without-noise.mp3 -c:v copy -map 0:v:0 -map 1:a:0 new-mp4.mp4
At first, it looked great! So I started doing it to every recording. Until I notice that the audio was progressively getting out of sync. That is the moment I notice that when I first extract the MP3 from the original file, this is returned way shorter. I spent a some hours trying to solve this problem, match the sample rate, bitrate and even stretching the audio didn't work (weird).
The worst part of this is that I noticed it too late. For most of the recording I already deleted the original file so I was trying really hard to find a way of solving this problem on the final file. If I am not able to fix it, I will have to record the tapes all over again (43+ hours of tapes).
Any help is appreciated. Thanks for your time in advance.
-af aresample=async=1and check.